In telecommunication systems, the phone handset or headset microphone is susceptible to common mode interference, such as power line interference noise signals or other electromagnetic radiation detected as noise or interference. This interference is commonly referred to as buzz and hum noise. The noise signals are fundamentally at 50-60 Hz and can also be associated with higher order frequency harmonics.
In the prior art, microphones use a metal enclosure to provide a measure of shielding from buzz and hum noise. Typically, the shielding is partially accomplished by mounting the microphone element in a cylindrical metal case or “can”. The back of a noise canceling microphone is typically a printed circuit board (PCB) having two layers of copper. The conductive case provides shielding from the sides, but does not shield the back of the microphone. In the prior art, careful PCB design to obtain maximum ground (at zero voltage reference) coverage using the two copper layers provided improved shielding from the back side and thus improved noise immunity. However, the PCB metal traces forming the copper layers still do not typically provide a contiguous grounded copper shield due to normal gaps between traces. As a result of these gaps, which consist of non-conductive material, capacitive coupling through the PCB occurs for both omni-directional and noise-canceling microphones.
Where the PCB contains an acoustic port, as in noise-canceling microphones, this capacitive coupling is increased further. Referring to FIG. 11, a prior art noise canceling microphone assembly 1100 is illustrated. The microphone assembly includes a housing can 1101 (also referred to herein as a microphone case), a printed circuit board (PCB) 1110, and a microphone transducer. The microphone transducer is typically an Electret type microphone comprised of a charged metallized diaphragm 1104 forming one plate of a capacitor and a backplate 1106 forming the other plate with a dielectric disposed in between. The charge is typically provided by an Electret material disposed on the surface of the back plate. The dielectric consists of an air gap 1102 between diaphragm 1104 and backplate 1106. Sound impinging on the diaphragm causes the diaphragm to vibrate. Diaphragm vibration varies the capacitance and produces a voltage signal proportional to the pressure difference across the diaphragm. Such Electret microphones typically use an integrated circuit (IC) 1108 having a junction field effect transistor (JFET) disposed on a printed circuit board (PCB) 1110 to amplify the output of the Electret microphone and transform the very high impedance of the small capacitor formed by the Electret microphone to a more usable lower value without undue capacitive divider losses. The microphone backplate 1106 is coupled to the gate terminal of the junction field effect transistor. Prior art noise canceling microphone assembly 1100 has a primary port 1112 (also referred to herein as a front port) in a front surface and a cancellation port 1114 (also referred to herein as a back or rear port) in the back surface of the housing in the PCB 1110. The noise cancellation port 1114 extends from a first side of PCB 1110 to a second side of PCB 1110.
In one example of a prior art device, the housing can 1101 has an open end 1103 with PCB 1110 forming the face of the open end 1103. Face 1105 of PCB 1110 with terminals 1116, 1118 forms the external surface of open end 1103. Noise cancellation port 1114 is used to cancel out undesired ambient or background noise which arrives from a different angle and originates much farther from the microphone than the voice of the user. Sound waves that arrive at opposite sides of the diaphragm in equal phase and amplitude do not induce diaphragm vibration. This condition is referred to as acoustic cancellation. In headset applications, the microphone/boot assembly is oriented such that sound waves emanating from the desired sound source (user's mouth) reach the front face of the diaphragm earlier and with greater amplitude than they reach the rear face of the diaphragm. Thus, acoustic cancellation is minimized. In contrast, sound waves emanating from sound sources that are located far away and in other directions arrive at opposite sides of the diaphragm in more nearly the same phase and amplitude, resulting in more acoustic cancellation. Therefore, the microphone is less sensitive to ambient noise than to the user's voice. This phenomenon is referred to as “noise cancellation”.
Thus, noise canceling Electret microphone 100 requires one or more acoustic ports on the back of the microphone. Since this acoustic port and the PCB output terminals cannot be easily covered with low cost PCB grounded shielding solutions, the overall shielding is compromised such that highly sensitive self-capacitance elements within the microphone are exposed to external noise influence. This electrical noise coupling is generally capacitive in nature. There are also forms of electromagnetic coupling from strong local radio frequency interference.
In the prior art, best practices in Electret microphone design and cable shielding could only reduce the hum and buzz from common mode interference (CMI) by roughly 6 to 10 dB. This proved to be not enough for host phone systems designed with high power line leakage potentials. Detectable low level audio hum and buzz continued to plague these microphone design efforts.
Thus, there is a need for improved methods and systems for electrical and electromagnetic interference shielding of Electret microphones without affecting acoustic operation of the microphone.